As shown in FIG. 1, a speed governor 1 of a mechanical timepiece is generally composed of an escape wheel 10, an anchor striker 20 and a balance 30. To the balance 30, a hairspring 40 that is a thin plate spring in the form of a spiral having a plural winding number is fixed. Specifically, the inner end of the hairspring 40 is fixed to a collet mounted on a balance staff that is a center shaft of the balance 30, and the outer end of the hairspring 40 is fixed to a stud 42. In the mechanical timepiece, by the action of this hairspring 40, repetitive movement transmitted from the anchor striker 20 is converted to a constant period of regular oscillation, whereby the rotation of the balance 30 can be controlled.
As a material of such a hairspring, a constant elastic alloy having an elinvar effect that the change of Young's modulus with temperature is extremely small, such as an alloy wherein Ti has been added to a Fe—Ni—Cr alloy, is known (see patent literature 1). Specifically, a hairspring material composed of an alloy consisting of 40.0 to 44.5% by weight of Ni, 4.50 to 6.50% by weight of Cr, 1.50 to 3.50% by weight of Ti, not more than 0.80% by weight of Al, not more than 2.50% by weight of total of C, Mn, Si, S and P, and a remainder that is Fe is described in the patent literature 1.
The alloy disclosed in this patent literature 1 is slightly distorted by magnetostriction due to self magnetization, and a change that the interatomic distance is decreased by reduction of magnetization with temperature increase and a change that the interatomic distance is increased by thermal expansion are compensated each other. On this account, the change of Young's modulus with temperature in a given temperature region (not higher than Curie temperature) becomes extremely small. The temperature coefficient of Young's modulus is a value obtained by quantifying a change of Young's modules with temperature increase, and it indicates a quantity of change of Young's modulus per ° C.
The present inventors have used in the past a hairspring material composed of an alloy which is different from the alloy disclosed in the patent literature 1 in chemical components and which contains 37.5 to 39.5% by mass of Ni, 9.2 to 9.9% by mass of Cr, 0.35 to 0.55% by mass of Ti, 0.6 to 0.9% by mass of Be and 0.06 to 0.12% by mass of C (carbon), based on the total amount, and contains a remainder of Fe and unavoidable impurities, said alloy being an alloy containing, as the unavoidable impurities, not more than 0.5% by mass of Mn and less than 0.03% by mass of Al. After the final solution treatment, cold drawing, cold rolling and aging treatment have been appropriately applied to this material, whereby a hairspring having not only proper strength and Young's modulus but also proper temperature coefficient of Young's modulus has been produced. When the temperature coefficient of Young's modulus of a hairspring is controlled to a proper range, there is an advantage that a variation in moment of inertia of a balance wheel caused by temperature change can be corrected.
The hairspring material having been used by the present inventors has an extremely small change of Young's modulus with temperature, but a variation in Young's modulus of a hairspring obtained from this material is sometimes observed, so that there is yet room for improvement.